Final answer:
The observation that proves a cell is a eukaryote is the presence of a nucleus.
Step-by-step explanation:
The observation that proves that a cell is a eukaryote is that it has a nucleus. In eukaryotic cells, the nucleus houses the genetic material (DNA) and is surrounded by a nuclear membrane. This differentiates eukaryotic cells from prokaryotic cells, which lack a true nucleus.
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